[Biomechanics in the pathogenesis of spondylosis and spondylolisthesis].

The etiology and predisposing factors of spondylolysis and spondylolisthesis, which are summarized in the first part of this article, have received considerable attention, but there biomechanics is little published. The goal of this article is to show a biomechanical model for the development of spondylolysis and spondylolisthesis. With mechanical equations it has been established, the pars interarticularis and the ligaments resist together the tensile and shear force, the bending moment if the pars interarticularis is uncracked. If the tensile stress in the pars interarticularis reaches its strength, crack occurs and the spondylolysis is developed. The cracked pars interarticularis is no longer capable of sustaining tension, the tensile force is transferred to the ligament. When the compressive strain of the pars interarticularis reaches its strain limit, the spondylolisthesis does not develop, because the vertebra can not slip with the unbroken ligaments. If the loading on the pars interarticularis would be decreasing, the cracks close and the pars interarticularis can ossify. If the tensile stress in the ligament reaches its strength and the ligament breaks, the pars interarticularis cracks through, the vertebra slips and the spondylolisthesis develops. In the last part of the article the methods of the conservative and operative treating are summarized.